This is folk tale in Tibet.
Amitābha saw and sad from the western pure realm that all sentient beings who suffer in samsara so he sent Avalokiteśvara to save them. But after know that there are so many people to save, Amitābha gave him to eleven heads to help her cries of those who are suffering. But still there are too many people need to save so Avalokiteśvara’s body shattered into pieces after work too much. So Amitābha gave him more strong body which has one thousand arms and eyes.
Nowadays he is known as manifestation of Avalokiteśvara to save all sentient beings by vast power.